Left Turn Phasing

The protected left turn phase provides a green arrow for left turning vehicles while stopping both on-coming traffic and parallel pedestrian crossings to eliminate conflicts. Signal operators and designers should consider the possibility that pedestrians will assume it is safe to cross during the protected left turn phase because the cross-street is still receiving a red signal indication. Protected left turn phasing is particularly appropriate for locations with relatively high left turn volumes.

An alternative signal phasing option is a protected/permissive phase, which provides a protected period for left turning vehicles either preceded or followed by a permissive left turn phase. A protected/ permissive left turn phase only partially eliminates conflicts between left turning vehicles and pedestrians using the parallel crosswalk. This option may be appropriate where left turn traffic volumes are relatively manageable but opposing through volumes are relatively high. A “yellow trap” conflict may result from protected/permissive left turn phasing when the signal indication for left turning vehicles changes from green to yellow/red but opposing traffic continues to receive a green indication. In this case, a flashing yellow arrow turn signal would be appropriate to prevent the “yellow trap.”

Implementing protected left turn phasing may reduce intersection vehicle capacity, impact signal system coordination, or require longer cycle lengths to manage combined vehicle and pedestrian traffic volumes. Related roadway geometry modifications to support protected left turn phasing include exclusive left turn lanes. Opposing left turn lanes may be provided at intersections without widening a roadway by converting existing median, two-way left turn lane, or travel lanes.

Purpose

One of the most common conflicts at signalized intersections is the competition between vehicles permissively turning left and pedestrians crossing during the concurrent pedestrian signal phase. Drivers typically focus on on-coming traffic to identify gaps for left turns and may not pay due attention to pedestrians approaching or in the parallel crosswalk. Furthermore, permissive left turns at congested intersections contribute to drivers accepting smaller gaps, turning at higher speeds, and “sneaking” through the intersection during the yellow or all-red signal intervals.

Estimated Cost

Adjusting signal phasing/timing settings is very low cost and requires a few hours of staff time to accomplish. New signal equipment can range from $8,000 to $150,000, depending on what items are needed.

Safety Effects

A summary of studies that have looked at the safety effects of left turn phasing can be found here.